The present invention relates generally to a fault detector to detect ground faults in a power distribution system. More specifically, the invention relates to the detection of ground faults using one or more isolated current sensors that measure the difference in the current supplied to the load(s) and the current returning from the load(s).
One type of power delivery system is an isolated power source that provides power to one or more loads where the power source, the power delivery system, and the load are all isolated from earth ground. A fault from the power system to an earth ground can occur in the power source, the power delivery system, or the load. Under some circumstances, this fault can be difficult to detect. However, if a second fault occurs anywhere within the system a high current circuit can be created through the ground path. This high current circuit can lead to personal injury and/or fire hazards.
Power over Ethernet (POE) systems are isolated power systems in which a common 48 volt power source provides power to distributed loads over the same infrastructure wiring that carries the Ethernet data. Complete isolation from ground or other power systems is required for the system to operate. As discussed above, single isolation faults in POE isolated power systems can go undetected, with second faults potentially leading to hazardous conditions. Typically, low side isolation faults, i.e., faults in the return path from the load to the power source, are easily detected and low side detection circuits are known in the art and are often included within an integrated circuit. High side fault detection, i.e., faults in the power supply path from the power source to the load, have unique problems. High side fault detection requires circuitry that is both accurate and tolerant of voltage standoff issues when integrated using standard BiCMOS processes.
A typical solution to high side fault detection and isolation is to apply an external self-resetting PTC fuse in the high side path. Although this provides isolation protection, the aging effects on the PTC fuse due to over-current levels reduce the accuracy and long-term repeatability of the PTC fuse to sense and protect high side isolation faults. In addition, a PTC fuse does not provide any indication to the operator as to the location of the fault. In addition, the use of individual PTC fuses in a multi-channel system can excessively increase the cost of the overall system.
Therefore, it would be advantageous to provide an isolation loss detector that operates on both the high side and the low side of the power delivery system, provides indicia as to the location of the isolation loss, and further does not deteriorate over time.